Hydraulic Cranes. 741 whole apparatus is fixed to the crane by feet j j. A wire rope or chain being attached to the eyebolt K, and carried round the pullies E1D1E2D2E3, leaves D3, by w, to the load or slewing: wheel, as desired. Examining by the pulley principle Fig. 439, p. 483, the mechanical advantage will be inversely as the number of cords or chains at L L, Fig. 729, p being now the greater, and w the lesser force. Neglecting friction, W___!___ P "~ no. of cords Mech. Adv. = — And allowing for all resistances, • P W no. of cords where efficiency 77 varies with the number of pullies, by the following table, found from practice : VALUES OF ?? FOR HYDRAULIC CRANES. No. OF PULLIES. o 2 4 6 8 10 12 14 16 77 = •87 •8 •76 •72 -67 •63 "59 '54 '5 and the greater tension, at tail end, equals P •»» (no. of cords x 17)* Thus, a heavy pressure with slow speed has lifted a smaller load at greater speed, the distance between pulley centres having been increased. In order that the ram shall finish its stroke quietly, automatic cut-off gear is supplied. Valve H being opened to pressure by raising rod N fully, the ram, ascending, strikes a tappet R by means of the projection Q, when the stroke is nearly complete, thus causing lever M to be pulled over to position s, and closing the valve. A further movement of M to position T opens H to exhaust, and the ram descends by the pull of the load, Reference may now be made to Plates XV. and XVLy shewing various hydraulic cranes. That on the left in Plate XV. is the best example of pulley gear. Thus, cylinder D is for lifting, E for traversing, and c for slewing, all worked from valves at s.